The present disclosure relates generally to dual or multiple energy imaging and in particular, to a detector acquisition sequence and system for dual or multiple energy imaging to minimize artifacts and total exam time between acquisitions.
The classic radiograph or “X-ray” image is obtained by situating the object to be imaged between an X-ray emitter and an X-ray detector made of photographic film. Emitted X-rays pass through the object to expose the film, and the degree of exposure at the various points on the film are determined by the density and thickness of the object along the path of the X-rays.
X-ray images may be used for many purposes. For instance, internal defects in a target object may be detected. Additionally, changes in internal structure or alignment may be determined. Furthermore, the image may show the presence or absence of objects in the target. The information gained from x-ray imaging has applications in many fields, including medicine and manufacturing.
It is now common to utilize solid-state digital X-ray detectors (e.g., an array of switching elements and photo-sensitive elements such as photodiodes) in place of film detectors. The charges generated by the X-rays on the various points of the detector are read and processed to generate a digital image of the object in electronic form, rather than an analog image on photographic film. Digital imaging is advantageous because the image can later be electronically transmitted to other locations, subjected to diagnostic algorithms to determine properties of the imaged object, and so on.
One embodiment of a solid state digital x-ray detector may be comprised of a panel of semiconductor Field Effect Transistors (FETs) and photodiodes. The FETs and photodiodes in the panel are typically arranged in rows (scan lines) and columns (data lines). A FET controller controls the order in which the FETs are turned on and off. The FETs are typically turned on, or activated, in rows. When the FETs are turned on, charge to establish the FET channel is drawn into the FET from both the source and the drain of the transistor. Due to the imperfect nature of the amorphous silicon FETs, the charge is retained temporarily when the FET is turned off and bleeds out, decaying, over time, which corrupts the desired signal in the form of an offset. The source of each FET is connected to a photodiode. The drain of each FET is connected to read-out electronics via data lines. Each photodiode integrates the light signal and discharges energy in proportion to the x-rays absorbed by the detector. The gates of the FETs are connected to the FET controller. The FET controller allows signals discharged from the panel of photodiodes to be read in an orderly fashion. The read-out electronics convert the signals discharged from photodiodes. The energy discharged by the photodiodes in the detector and converted by the read-out electronics is used by an acquisition system to activate pixels in the displayed digital diagnostic image. The panel of FETs and photodiodes is typically scanned by row. The corresponding pixels in the digital diagnostic image are typically activated in rows.
The FETs in the x-ray detector act as switches to control the charging and discharging of the photodiodes. When a FET is open, an associated photodiode is isolated from the read-out electronics and is discharged during an x-ray exposure. When the FET is closed, the photodiode is recharged to an initial charge by the read-out electronics. Light is emitted by a scintillator in response to x-rays absorbed from the source. The photodiodes sense the emitted light and are partially discharged. Thus, while the FETs are open, the photodiodes retain a charge representative of the x-ray dose. When a FET is closed, a desired voltage across the photodiode is restored. The measured charge amount to re-establish the desired voltage becomes a measure of the x-ray dose integrated by the photodiode during the length of the x-ray exposure.
One source of difficulty faced by digital x-ray systems is the photoconductive characteristics of semiconductor devices used in the digital x-ray systems. Photoconductivity is an increase in electron conductivity of a material through optical (light) excitation of electrons in the material. Photoconductive characteristics are exhibited by the FETs used as switches in solid state x-ray detectors. Ideally, FET switches isolate the photodiode from the electronics, which measure the charge restored to the photodiode. FETs exhibiting photoconductive characteristics do not completely isolate the photodiode from the system, when the FETs are open. Consequently, the FETs transfer excess charge to the read-out electronics. If the FETs transfer excess charge to the read-out electronics, the energy subsequently discharged from the photodiodes to activate the pixels in the digital image may be affected. The unintended charge leakage through the FETs may produce artifacts or may add a non-uniform offset value to each of the pixels in the digital x-ray image, thus producing a line artifact in the image.
FETs and other materials made of amorphous silicon also exhibit a characteristic referred to as charge retention. Charge retention is a structured phenomenon and be controlled to a certain extent. Charge retention corresponds to the phenomenon whereby not all of the charge drawn into the FET to establish a conducting channel is forced out when the FET is turned off. The retained charge leaks out of the FET over time, even after the FET is turned off, and the leaked charge from the FET adds an offset to the signal read out of the photodiodes by the x-ray control system.
The FETs in the x-ray detector exhibit charge retention characteristics when voltage is applied to the gates of the FETs to read the rows of the x-ray detector. The detector rows are generally read in a predetermined manner, sequence, and time interval. The time interval may vary between read operations for complete frames of the x-ray image. When a FET is opened after the charge on an associated photodiode is read by a charge measurement unit, the FET retains a portion of the charge. Between read operations, the charge retained by the FETs leaks from the FETs to a charge measurement unit. The amount of charge that leaks from the FETs exponentially decays over time. The next read operation occurs before the entire retention charge leaks from the FETs. Consequently, the charge measurement unit measures during each read operation an amount of charge that is retained by the FETs during the read operation for the present scan line. The charge measurement unit also reads an amount of charge that was stored by FETs that were activated in scan lines preceding (in time) the current scan line in both the current (detector) read operation and the preceding (detector) read operation.
The charge leaking from the FETs when a new read operation is initiated is referred to as the initial charge retention. The initial charge retention stored on multiple FETs, such as the FETs of a single data line, combines to form a charge retention offset for that column. The charge retention offset varies based on the rate at which rows of the x-ray detector panel are read. As the interval increases between read operations, the charge decay increases. As the panel rows are read, the charge retention offset builds to a steady state value. The steady state value for the charge retention rate represents the point at which the panel rows are read at a rate equaling the exponential decay rate of the charge on the FETs.
During an x-ray exposure, a similar phenomenon occurs whereby charge is generated in the FET as a result of the FET photoconductive characteristics. When the FETs are turned off at the end of the exposure, the additional charge also leaks out and adds to the read signal in a manner analogous to charge retention. However, the additional charge cannot be removed because the additional charge, resulting from the FET photoconductive characteristics, relates to the x-rays bombarding the x-ray detector. The number of FETs that photoconduct and the amount of charge conducted by the FETs are dependent upon the amount of x-ray exposure and the object imaged, as well as upon the individual properties of each FET. Since a solid state x-ray detector is structured along rows (scan lines) and columns (data lines), the excess charge in the FETs may result in structured image artifacts (e.g., pixels, lines, shapes) or offsets.
During the digital imaging process, the image is generally not produced directly from the detector reading. Instead, the detector reading is processed to produce a cleaner image. In particular, the image is usually processed to eliminate the “offset”, which arises owing to the photoconductive characteristics of the detector prior to the time the exposure is made. The qualities of the offset are determined by the detector's current leakage, temperature, background radiation, and a variety of other factors. The offset is desirably eliminated from the detector reading to provide better image quality.
In dual-energy imaging, two sequential X-ray acquisitions are made using different X-ray spectra to produce a bone only and a soft tissue only image, which enhances the visualization of nodules and calcification. The X-ray spectrum is modified by the X-ray generator energy and/or spectral filters being utilized. To minimize patient motion between the two acquisitions, they are acquired as close together as possible in time. Patient motions between the two acquisitions will cause artifacts in the dual-energy images.
For a digital radiographic (DR) system operated in a dual energy mode, the detector must be read between the two exposures. The “normal” radiographic acquisition sequence includes: a first X-ray dosage (Xray1), a first delay following the first X-ray dosage (Delay1) a first read following the first delay (Read1), a first X-ray dosage following the first read (Xray2), a second delay following the second X-ray dosage (Delay2), and a second read following the second delay (Read2). Typically, a number of detector “scrubs” are made between exposures Xray1 and X-ray2. These scrubs are a detector reading without x-ray exposure, and serve to maintain the electrical stability of the amorphous silicon FETs and reduce the image lag.
To ensure that the detector is read as fast as possible without artifacts due to patient motion, the e-ray acquisitions should be as close as possible. To ensure that the X-ray acquisitions are as close together as possible, the critical time to minimize is equal to the sum of the duration of Xray1, the duration of Delay1, the duration of Read1, and the duration of Xray2. Typical values for these parameters are: Xray1 50 milliseconds, Delay1 50 milliseconds, Read1 80 milliseconds, and Xray2 50 milliseconds. Thus, the estimate of this critical time is 50+50+180+50=320 milliseconds. Clinical and research studies have suggested that a critical time of 50 milliseconds is needed to freeze the motion of the heart and the pressure wave that conducts through the lungs during a dual-energy chest exam in order to completely eliminate artifacts due to patient motion.
Other medical imaging applications where this critical time is of importance includes bone mineral densitometry (BMD) and tomosynthesis. Although BMD uses the dual energy application, spatial resolution is not critical because the current state of the art systems use a 1 mm2 pixel pitch. Tomosynthesis is performed by acquiring multiple images with a digital detector, i.e. series of low dose images used to reconstruct tomography images at any level. Tomosynthesis may be performed using many different tube motions including linear, circular, elliptical, hypocycloidal, and others. In tomosynthesis, image sequences are acquired, with typical number of images ranging from 5 to 50.
Fast acquisition techniques in general have the propensity to cause artifacts in solid-state detectors. With any image acquisition, artifacts must be minimized and/or eliminated to prevent an impairment of the diagnostic value of the images. These artifacts include, but are not limited to pixels, lines, and shapes artifacts. Examples of the causes of these artifacts include lag, gain hysteresis, and timing mode changes. These artifacts may appear in any of the x-ray images or offset images. As such, the fast imaging read scenario must optimize the critical time without introducing any image artifacts.